U.S. patent application number 12/265929 was filed with the patent office on 2009-03-05 for method and apparatus for estimating and correcting baseband frequency error in a receiver.
This patent application is currently assigned to INTERDIGITAL TECHNOLOGY CORPORATION. Invention is credited to Mihaela Beluri, Alpaslan Demir, Philip J. Pietraski, Rui Yang.
Application Number | 20090060020 12/265929 |
Document ID | / |
Family ID | 36316290 |
Filed Date | 2009-03-05 |
United States Patent
Application |
20090060020 |
Kind Code |
A1 |
Pietraski; Philip J. ; et
al. |
March 5, 2009 |
METHOD AND APPARATUS FOR ESTIMATING AND CORRECTING BASEBAND
FREQUENCY ERROR IN A RECEIVER
Abstract
A method and apparatus for estimating and correcting baseband
frequency error in a receiver. In one embodiment, an equalizer
performs equalization on a sample data stream and generates filter
tap values based on the equalization. An estimated frequency error
signal is generated based on at least one of the filter tap values.
A rotating phasor is generated based on the estimated frequency
error signal. The rotating phasor signal is multiplied with the
sample data stream to correct the frequency of the sample data
stream. In another embodiment, a channel estimator performs channel
estimation and generates Rake receiver finger weights based on at
least one of the finger weights. An estimated frequency error
signal is generated based on at least one of the finger
weights.
Inventors: |
Pietraski; Philip J.;
(Huntington Station, NY) ; Yang; Rui; (Greenlawn,
NY) ; Beluri; Mihaela; (Huntington, NY) ;
Demir; Alpaslan; (East Meadow, NY) |
Correspondence
Address: |
VOLPE AND KOENIG, P.C.;DEPT. ICC
UNITED PLAZA, SUITE 1600, 30 SOUTH 17TH STREET
PHILADELPHIA
PA
19103
US
|
Assignee: |
INTERDIGITAL TECHNOLOGY
CORPORATION
Wilmington
DE
|
Family ID: |
36316290 |
Appl. No.: |
12/265929 |
Filed: |
November 6, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11209097 |
Aug 22, 2005 |
7457347 |
|
|
12265929 |
|
|
|
|
60625874 |
Nov 8, 2004 |
|
|
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Current U.S.
Class: |
375/232 |
Current CPC
Class: |
H04B 1/7097 20130101;
H04B 1/712 20130101; H04L 25/03248 20130101 |
Class at
Publication: |
375/232 |
International
Class: |
H04L 27/01 20060101
H04L027/01 |
Claims
1. A frequency error estimator configured to generate an estimated
frequency error signal based on at least one of a plurality of
filter tap values, the frequency error estimator comprising: a tap
extraction unit for extracting an appropriate tap value from the
filter tap values generated by the equalizer and outputting the
extracted tap value; a delay unit coupled to the tap extraction
unit for delaying the extracted tap value; a conjugate generator
coupled to the tap extraction unit and the delay unit for
generating a conjugate of the extracted tap value; a second
multiplier coupled to the delay unit and the conjugate generator
for multiplying the conjugate of the extracted tap value with the
delayed extracted tap value to generate a phase difference signal
which represents the phase difference between the conjugate
extracted tap value and the delayed extracted tap value; an
arctangent unit coupled to the output of the second multiplier for
measuring the phase difference between the conjugate extracted tap
value and the delayed extracted tap value; a first filter for
averaging the measured phase difference, whereby the averaged phase
difference is output from the frequency error estimator as the
estimated frequency error signal; and a comparator circuit coupled
to the second multiplier and the first filter for selectively
sending a hold signal to the first filter based on a value of a
threshold signal, wherein the hold signal causes the first filter
to pause.
2. The frequency error estimator of claim 1 wherein the comparator
circuit comprises: a magnitude detector coupled to the second
multiplier and the arctangent unit for calculating a magnitude of
the phase difference signal; and a comparator for comparing the
magnitude of the phase difference signal with the value of the
threshold signal and generating the hold signal if the magnitude of
the phase difference signal is below the value of the threshold
signal.
3. The frequency error estimator of claim 2 wherein the comparator
circuit further comprises: a second filter for averaging the
magnitude of the phase difference signal; and a third multiplier
for multiplying a scaling factor with the averaged magnitude of the
phase difference signal to generate the threshold signal.
4. The frequency error estimator of claim 1 wherein the comparator
circuit comprises: a power detector for calculating instantaneous
power of the phase difference signal; and a comparator for
comparing the instantaneous power of the phase difference signal
with a value of a threshold signal, whereby the comparator
generates the hold signal if the instantaneous power of the phase
difference signal is below the value of the threshold signal.
5. The frequency error estimator of claim 4 wherein the comparator
circuit further comprises: a second filter for averaging the
instantaneous power of the phase difference signal; and a third
multiplier for multiplying a scaling factor with the averaged
instantaneous power of the phase difference signal to generate the
threshold signal.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/209,097 filed Aug. 22, 2005, which claims
the benefit of U.S. provisional application No. 60/625,874 filed
Nov. 8, 2004, which is incorporated by reference as if fully set
forth.
FIELD OF INVENTION
[0002] The present invention is related to wireless receivers. More
particularly, the present invention is related to a method and
apparatus for estimating and correcting frequency error at baseband
in a receiver.
BACKGROUND
[0003] Adaptive receivers, such as a normalized least mean square
(NLMS) equalizer used in wireless transmit/receive units (WTRUs)
and base stations, optimize their associated filter tap values
through an iterative procedure that requires multiple iterations to
near convergence. The tap values converge as time passes to a
minimum mean square error (MMSE) solution used to perform channel
estimation.
[0004] An NLMS receiver includes an equalizer having an equalizer
filter which is continually in the process of converging as it
tries to track a time-varying channel. The more complex it is to
track the channel, the further the tap values of the equalizer will
be from convergence. Generally, faster channels, (i.e., channel
states that evolve rapidly), are difficult for the equalizer to
track. Residual automatic frequency control (AFC) errors in the
baseband input into the equalizer cause channels to appear faster
than they really are. The increase in the apparent speed of the
channel can only be partially mitigated by increasing the step-size
of an NLMS algorithm implemented by the NLMS receiver. The
increased step-size allows the equalizer filter to more accurately
track "fast" channels, but it also increases errors in the MMSE
solution which cause degradation in the performance of the
receiver.
[0005] Receivers that employ channel estimation are also degraded
by residual AFC errors. Since the bandwidth of the appropriate
equalizer filter used in channel estimation is a function of the
apparent speed of the channel, large AFC errors force the use of
wide-band filters that do not efficiently suppress noise, thus
leading to less accurate channel estimates. A simple solution is
desired to suppress the residual AFC errors.
SUMMARY
[0006] The present invention is related to a method and apparatus
for estimating and correcting baseband frequency error in a
receiver. In one embodiment, an equalizer performs equalization on
a sample data stream and generates filter tap values based on the
equalization. An estimated frequency error signal is generated
based on at least one of the filter tap values. A rotating phasor
is generated based on the estimated frequency error signal. The
rotating phasor signal is multiplied with the sample data stream to
correct the frequency of the sample data stream. In another
embodiment, a channel estimator performs channel estimation and
generates Rake receiver finger weights based on at least one of the
finger weights. An estimated frequency error signal is generated
based on at least one of the finger weights.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] A more detailed understanding of the invention may be had
from the following description, given by way of example and to be
understood in conjunction with the accompanying drawings
wherein:
[0008] FIG. 1 is a block diagram of an exemplary BFC system
including a frequency error estimator for removing residual AFC
errors in accordance with one embodiment of the present
invention;
[0009] FIG. 2 is a block diagram of the frequency error estimator
of the system 100 of FIG. 1;
[0010] FIG. 3 is a block diagram of an exemplary BFC system in
accordance with another embodiment of the present invention;
[0011] FIG. 4 is a high level flow diagram of a process for
correcting the frequency of a sample data stream in a wireless
communication receiver having an equalizer that performs
equalization in accordance with one embodiment of the present
invention;
[0012] FIG. 5 is a flow diagram of a process for generating the
estimated frequency error signal based on based on a filter tap
value extracted from filter tap values generated by the equalizer
used in the process of FIG. 4;
[0013] FIG. 6 is a flow diagram of a process for generating the
estimated frequency error signal based on a plurality of extracted
tap values that are averaged in accordance with one embodiment of
the present invention;
[0014] FIG. 7 is a flow diagram of a process for comparing the
magnitude of the phase difference signal with the value of the
threshold signal to determine whether the estimated frequency error
signal should be prevented from being updated in accordance with
one embodiment of the present invention;
[0015] FIG. 8 is a flow diagram of a process for comparing the
instantaneous power of the phase difference signal with the value
of the threshold signal to determine whether the estimated
frequency error signal should be prevented from being updated in
accordance with one embodiment of the present invention;
[0016] FIG. 9 is a high level flow diagram of a process for
correcting the frequency of a sample data stream in a wireless
communication receiver having a channel estimator that performs
channel estimation in accordance with one embodiment of the present
invention; and
[0017] FIG. 10 is a flow diagram of a process for generating the
estimated frequency error signal based on a finger weight extracted
from Rake receiver finger weights generated by the channel
estimator used in the process of FIG. 9.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] The preferred embodiments will be described with reference
to the drawing figures where like numerals represent like elements
throughout.
[0019] Hereafter, the terminology "WTRU" includes but is not
limited to a user equipment (UE), a mobile station, a laptop, a
personal data assistant (PDA), a fixed or mobile subscriber unit, a
pager, or any other type of device capable of operating in a
wireless environment. When referred to hereafter, the terminology
"base station" includes but is not limited to an access point (AP),
a Node-B, a site controller or any other type of interfacing device
in a wireless environment.
[0020] The features of the present invention may be incorporated
into an integrated circuit (IC) or be configured in a circuit
comprising a multitude of interconnecting components.
[0021] Hereinafter, the present invention will be described in
terms of the NLMS equalizer. However, it should be noted that the
NLMS equalizer based receiver is provided as an example and the
present invention can be applied to receivers implementing any
other adaptive equalization algorithm and to receivers employing
channel estimation such as block based equalizers and rake
receivers.
[0022] FIG. 1 is a block diagram of an exemplary BFC system 100 for
removing residual AFC errors in accordance with one embodiment of
the present invention. The BFC system 100 may be incorporated in a
WTRU or a base station. The BFC system 100 includes a multiplier
102, an equalizer 104, a frequency error estimator 106, a
controller 108 and a numerically controlled oscillator (NCO) 110.
The equalizer 104 processes a sample data stream 112 provided by a
receiver front end (not shown) via the multiplier 102. The
equalizer may operate in accordance with an NLMS algorithm.
However, any other type of adaptive equalizer algorithm may be
applied.
[0023] Filter tap values 114 generated by the equalizer 104 are
provided as an input to the frequency error estimator 106. The
frequency error estimator 106 generates an estimated frequency
error signal 116. The residual frequency errors after AFC can be
greatly reduced by BFC based solely on observation of at least one
tap value in the equalizer 104, or alternatively from partial
channel estimates, such as a Rake finger complex weight estimation.
BFC is accomplished by estimating the frequency error based on
observation of the one or more taps in the equalizer 104,
generating a correction signal consisting of a complex sinusoid (or
rotating phasor), correcting the input samples data stream by
multiplying it by the phasor and applying frequency corrected
samples 118 to the input of the equalizer 104 in a closed loop
fashion.
[0024] The residual frequency error is estimated by periodically
measuring the phase change of one or more of the tap values of the
equalizer 104, (or alternatively, partial channel estimates). Much
of the phase change measured on the equalizer filter taps 114 from
sample to sample is due to noise and fading. However, phase changes
due to fading and noise are zero mean, (e.g., have a mean value of
zero). Therefore, the expected value of any sample average will be
zero, i.e., the average value of the signal is zero. Thus,
filtering can be used to remove noise and fading components which
cause phase change from the overall phase changes, (due to, e.g.,
residual AFC errors), and to recover the slowly varying phase
change due to the frequency error.
[0025] Once the frequency error is estimated by the frequency error
estimator 106, the controller 108 processes the estimated frequency
error signal 116 to generate a frequency adjustment signal 120. The
controller 108 may simply adjust the gain of the estimated
frequency error signal 116 or may process the estimated frequency
error signal 116 with a more complicated algorithm, (e.g., a
proportional-integral-derivative (PID)). The frequency adjustment
signal 120 is fed to the NCO 110 which generates a rotating phasor
122 which corresponds to the frequency adjustment signal 120. The
multiplier 102 multiplies the rotating phasor 122 with the sample
data stream 112 to generate the frequency corrected samples 118
input into the equalizer 104.
[0026] Residual AFC errors manifest themselves in the baseband as a
multiplicative error in the baseband signal and has the form of a
complex sinusoid, such as g(t)*exp(j*2pi*f*t) where g(t) is the
desired uncorrupted baseband signal and exp(j*2pi*f*t) is the
complex sinusoid representing the error. By multiplying by
exp(-j*2pi*f*t), the complex sinusoids cancel leaving only the
desired signal g(t). The estimated frequency error signal 116 is
input to the controller 108 which, in turn, outputs a signal 120
which may be, for example, a scaled, (i.e., proportional), version
of the input, e.g., four (4) times the value of the estimated
frequency error signal 116. The output signal 120 of the controller
108 may also include other terms such as a term proportional to the
integrals and/or derivatives of the estimated frequency error
signal 116. More generally, the output signal 120 could also be
clipped to be within some range or have other such non-linear
function applied to it. The NCO (110) takes as an input a frequency
value and outputs a constant magnitude complex signal with
instantaneous frequency equal to the value of the input, e.g.,
exp(j*2pi*f*t), where f is the input frequency.
[0027] FIG. 2 is a block diagram of the frequency error estimator
106 of the BFC system 100 shown in FIG. 1. The frequency error
estimator 106 includes a tap extraction unit 202, a delay unit 204,
a conjugate generator 206, multipliers 208, 210, an arctangent unit
212, a magnitude detector 214, an averaging filter 216, a phase
change filter 218 and a comparator 220. The equalizer generates
filter taps 114 which are supplied to the frequency error estimator
106.
[0028] In the frequency error estimator 106, the tap extraction
unit 202 extracts and outputs an appropriate tap value or average
of tap values onto an output signal 203 from the filter taps 114,
(or alternatively from a channel estimator), to use for performing
frequency estimation. For example, at least one appropriate tap
value corresponding to an FSP in a particular channel may be
extracted from the equalizer filter taps 114. The tap extraction
unit 202 may also track frequency drifting of the extracted tap
value.
[0029] The extracted tap value 203 is forwarded to a delay unit 204
and a conjugate unit 206. The delay unit 204 delays the extracted
tap value 203 for a predetermined period of time by outputting a
delayed tap value 205. The conjugate generator is used to generate
a conjugate 207 of the extracted tap value 203. The multiplier 208
multiplies the delayed tap value 205 by the conjugate tap value
207. The output 209 of the multiplier 208 has a phase value equal
to the phase difference between the delayed tap value 205 and the
conjugate tap value 207. This phase value is proportional to the
average frequency of the signal 203 and therefore of the sample
data stream 112.
[0030] The arctangent unit 212 measures an angle value 213 of the
output 209 of the multiplier 208. The angle value 213 is equal to
the phase difference between signal 205 and signal 207. Averaging
the angle value 213 is therefore equivalent to averaging the phase
difference between signal 205 and signal 207. The angle value 213
is filtered by the phase change filter 218 for averaging the angle
value 213. The measured average phase difference and the known
delay are used to generate the estimated frequency error signal
116.
[0031] For example, with a delay D (sec) and phase measured in
radians, the gain of the frequency error estimator 106 is
1/(2*PI*D). The "gain" refers to the conversion of a signal with a
net frequency error, (as indicated by signal 114), to an observed
value of the estimated frequency error signal 116. If the signal
114 has an average frequency of 1 Hz, then the output value on the
estimated frequency error signal 116 will be 1/(2*PI*D).
[0032] The magnitude detection unit 214 calculates the magnitude of
the output 209 of the multiplier 208 and sends a calculated
magnitude value 215 to a first input, X, of the comparator 220 and
to the averaging filter 216 for averaging. The multiplier 210
multiplies the output signal 217 of the averaging filter 216,
(i.e., the average value of signal 215), with a threshold factor
value 219, (e.g., a scaling factor having a value T), to generate a
threshold signal 222 which is sent to a second input, Y, of the
comparator 220. The value of the threshold signal 222 may be set to
a fraction of the average amplitude of the output 209 of the
multiplier 208. The threshold factor value, T, may be set, for
example, to 1/3. The comparator 220 compares the calculated
magnitude value 215 with the value of the threshold signal 222 and
sends a hold signal 221 to the phase change filter 218 if the
calculated magnitude value 215 is below the value of the threshold
signal 222.
[0033] The magnitude of the output 209 of the multiplier 208 may be
measured and compared to the average amplitude of the output 209 of
the multiplier 208, whereby the phase change filter 218 is paused
whenever the magnitude of the output 209 of the multiplier 208
drops below a threshold. When the filter 218 is paused, the
estimated frequency error signal 116 does not change, (i.e., the
signal 116 is not updated), the input 213 is not used, and the
internal state of the filter 218 does not change. The hold signal
221 is true whenever the signal 209 is relatively small. This has
the effect of discarding the angle values on signal 213 whenever
they are noisiest, and improving the estimated frequency error
signal 116 when the channel undergoes deep fades.
[0034] Alternately, a power detector (not shown) may be substituted
for the magnitude detector 214 to calculate the average power,
(i.e., the squared magnitude), of the output 209 of the multiplier
208, whereby the instantaneous power of the output 209 is compared
to some fraction of the average power. Other variations are also
possible.
[0035] FIG. 3 is a block diagram of a BFC system 300 for removing
residual AFC errors in accordance with another embodiment of the
present invention. The BFC system 100 may be incorporated in a WTRU
or a base station. The BFC system 300 includes a multiplier 302, a
rake combiner (or a block equalizer) 304, a channel estimator 306,
a frequency error estimator 308, a controller 310 and an NCO
312.
[0036] In a Rake receiver, a finger weight is determined based on
the channel estimation on a particular multipath component assigned
to a Rake finger. The channel estimator 306 generates Rake receiver
finger weights 316 which are provided as an input to the frequency
error estimator 308. The frequency error estimator 308 operates in
a fashion similar to the frequency error estimator 106 shown in
FIGS. 1 and 2.
[0037] Once the frequency error is estimated by the frequency error
estimator 308, the controller 310 processes the estimated frequency
error signal 318 to generate a frequency adjustment signal 322. The
frequency adjustment signal 322 is fed to the NCO 312 which
generates a rotating phasor 324 which corresponds to the frequency
adjustment signal 322. The multiplier 302 multiplies the rotating
phasor 324 with the sample data stream 314 to generate the
frequency corrected samples 320 input into the channel estimator
306 and the Rake combiner 304. Alternatively, a block equalizer may
be used instead of the Rake combiner 304.
[0038] The estimated frequency error signal 318 generated by the
frequency error estimator 308 is processed by the controller 310
and the NCO 312 which applies a rotating phasor 324 to the sample
data stream 314.
[0039] FIG. 4 is a high level flow diagram of a process 400
including method steps for correcting the frequency of a sample
data stream in a wireless communication receiver having an
equalizer that performs equalization in accordance with one
embodiment of the present invention. In step 405, equalization is
performed on a sample data stream. In step 410, filter tap values
are generated based on the equalization. In step 415, an estimated
frequency error signal is generated based on at least one of the
filter tap values. In step 420, a rotating phasor signal is
generated based on the estimated frequency error signal. In step
425, the rotating phasor signal is multiplied with the sample data
stream to correct the frequency of the sample data stream.
[0040] FIG. 5 is a flow diagram of a process 500 including method
steps for generating the estimated frequency error signal based on
a filter tap value extracted from filter tap values generated by
the equalizer used in process 400 of FIG. 4. In step 505, an
appropriate tap value is extracted from the filter tap values
(generated in step 410 of FIG. 4). In step 510, the extracted tap
value is delayed. In step 515, a conjugate of the extracted tap
value is generated. In step 520, the conjugate of the extracted tap
value is multiplied with the delayed extracted tap value to
generate a phase difference signal which represents the phase
difference between the conjugate extracted tap value and the
delayed extracted tap value. In step 525, the phase difference
between the conjugate extracted tap value and the delayed extracted
tap value is measured. In step 530, the estimated frequency error
signal is generated by averaging the measured phase difference. In
step 535, the estimated frequency error signal is selectively
prevented from being updated based on a value of a threshold
signal.
[0041] FIG. 6 is a flow diagram of a process 600 including method
steps for generating the estimated frequency error signal based on
a plurality of extracted tap values that are averaged in accordance
with one embodiment of the present invention. In step 605, a
plurality of tap values are extracted from the filter tap values
(generated in step 410 of FIG. 4). In step 610, the extracted tap
values are averaged to generate an average value of the extracted
tap values. In step 615, the average tap value is delayed. In step
620, a conjugate of the average tap value is generated. In step
625, the conjugate of the average tap value is multiplied with the
delayed average tap value to generate a phase difference signal
which represents the phase difference between the conjugate average
tap value and the delayed average tap value. In step 630, the phase
difference between the conjugate average tap value and the delayed
average tap value is measured. In step 635, the estimated frequency
error signal is generated by averaging the measured phase
difference. In step 640, the estimated frequency error signal is
selectively prevented from being updated based on a value of a
threshold signal.
[0042] FIG. 7 is a flow diagram of a process 700 including method
steps for comparing the magnitude of the phase difference signal
with the value of the threshold signal to determine whether the
estimated frequency error signal should be prevented from being
updated in accordance with one embodiment of the present invention.
In step 705, a magnitude of the phase difference signal is
calculated. In step 710, the magnitude of the phase difference
signal is averaged. In step 715, the threshold signal is generated
by multiplying a scaling factor with the averaged magnitude of the
phase difference signal. In step 720, the magnitude of the phase
difference signal is compared with the value of the threshold
signal. In step 725, the estimated frequency error signal is
prevented from being updated if the magnitude of the phase
difference signal is below the value of the threshold signal.
[0043] FIG. 8 is a flow diagram of a process 800 including method
steps for comparing the instantaneous power of the phase difference
signal with the value of the threshold signal to determine whether
the estimated frequency error signal should be prevented from being
updated in accordance with one embodiment of the present invention.
In step 805, the instantaneous power of the phase difference signal
is calculated. In step 810, the instantaneous power of the phase
difference signal is averaged. In step 815, the threshold signal is
generated by multiplying a scaling factor with the averaged
instantaneous power of the phase difference signal. In step 820,
the instantaneous power of the phase difference signal is compared
with the value of the threshold signal. In step 825, the estimated
frequency error signal is prevented from being updated if the
instantaneous power of the phase difference signal is below the
value of the threshold signal.
[0044] FIG. 9 is a high level flow diagram of a process 900
including method steps for correcting the frequency of a sample
data stream in a wireless communication receiver having a channel
estimator that performs channel estimation in accordance with one
embodiment of the present invention. In step 905, channel
estimation is performed on a sample data stream. In step 910, Rake
receiver finger weights are generated based on the channel
estimation. In step 915, an estimated frequency error signal is
generated based on at least one of the finger weights. In step 920,
a rotating phasor signal is generated based on the estimated
frequency error signal. In step 925, the rotating phasor signal is
multiplied with the sample data stream to correct the frequency of
the sample data stream.
[0045] FIG. 10 is a flow diagram of a process 1000 including method
steps for generating the estimated frequency error signal based on
a finger weight extracted from Rake receiver finger weights
generated by the channel estimator used in the process 900 of FIG.
9. In step 1005, an appropriate finger weight is extracted from the
Rake receiver finger weights (generated in step 910 of FIG. 9). In
step 1010, the extracted finger weight is delayed. In step 1015, a
conjugate of the extracted finger weight is generated. In step
1020, the conjugate of the extracted finger weight is multiplied
with the delayed extracted finger weight to generate a phase
difference signal which represents the phase difference between the
conjugate extracted finger weight and the delayed extracted finger
weight. In step 1025, the phase difference between the conjugate
extracted finger weight and the delayed extracted finger weight is
measured. In step 1030, the estimated frequency error signal is
generated by averaging the measured phase difference. In step 1035,
the estimated frequency error signal is selectively prevented from
being updated based on a value of a threshold signal.
[0046] Although the features and elements of the present invention
are described in the preferred embodiments in particular
combinations, each feature or element can be used alone without the
other features and elements of the preferred embodiments or in
various combinations with or without other features and elements of
the present invention.
* * * * *